Refactor tests directory structure (#894)

* Refactor tests directory structure to match library modules

- Created subdirectories in tests/: backend, executor, standalone, task_scheduler.
- Moved and renamed test files to corresponding subdirectories.
- Updated relative paths in test files for executables and backends.
- Added __init__.py files to new test packages.

Co-authored-by: jan-janssen <[email protected]>

* Update pipeline.yml

* Update pipeline.yml

* refactor

* fixes

* communication fix

* fix serial tests

* fix flux

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: jan-janssen <[email protected]>

Author: jan-janssen

.github/workflows/pipeline.yml

@@ -218,7 +218,7 @@ jobs:
       timeout-minutes: 5
       run: >
         flux start
-        python -m unittest tests/test_fluxpythonspawner.py tests/test_fluxjobexecutor_plot.py tests/test_fluxjobexecutor.py tests/test_fluxclusterexecutor.py;
+        python -m unittest tests/unit/task_scheduler/interactive/test_spawner_flux.py tests/unit/executor/test_flux_job_plot.py tests/unit/executor/test_flux_job.py tests/unit/executor/test_flux_cluster.py;
 
   unittest_flux_openmpi:
     needs: [black]
@@ -249,7 +249,7 @@ jobs:
       timeout-minutes: 5
       run: >
         flux start
-        coverage run -a -m unittest tests/test_fluxpythonspawner.py tests/test_fluxjobexecutor_plot.py tests/test_fluxjobexecutor.py tests/test_fluxclusterexecutor.py;
+        coverage run -a -m unittest tests/unit/task_scheduler/interactive/test_spawner_flux.py tests/unit/executor/test_flux_job_plot.py tests/unit/executor/test_flux_job.py tests/unit/executor/test_flux_cluster.py;
         coverage report;
         coverage xml
       env:
@@ -289,11 +289,11 @@ jobs:
       timeout-minutes: 5
       run: |
         pip install . --no-deps --no-build-isolation
-        cd tests
+        cd tests/unit
         sinfo -o "%n %e %m %a %c %C"
         srun --mpi=list
-        python -m unittest test_slurmjobexecutor.py
-        python -m unittest test_slurmclusterexecutor.py
+        python -m unittest executor/test_slurm_job.py
+        python -m unittest executor/test_slurm_cluster.py
 
   unittest_mpich:
     needs: [black]